In this article, we present a formal model of entropy reduction across the rounds when a fault is injected in SIMON and SIMECK family of lightweight ciphers. The model helps to pinpoint a range of intermediate rounds in a cipher of the same family, which when subjected to a fault injection requires minimal number of such attempts to reveal the secret key. The range of such rounds depict increased vulnerability to fault analysis attacks and, thus, require a stronger countermeasure for such rounds. We demonstrate the proposed entropy-reduction model for all versions of SIMON and SIMECK. The comparisons with existing fault analysis attacks depict that our proposed model requires least number of faults and smaller attack time in almost all versions of SIMON and SIMECK. The proposed entropy-reduction model can be used as a tool for the designers for any generic lightweight Feistel cipher to identify the vulnerable rounds in the encryption/decryption algorithms.